Inkjet printers generally incorporate multiple printheads in a scanning carriage which scans left-to-right and right-to-left across a medium while the printheads eject droplets of ink. The printheads are typically housed in one or more print cartridges either containing ink or having ink supplied to them from an external source. The ink is channeled to ink ejection chambers formed on a substrate associated with each printhead. Within each of the ink ejection chambers is an ink ejection element, such as a resistive heater or a piezoelectric element. A nozzle plate resides over each printhead such that each nozzle is aligned over an ink ejection chamber. Each printhead may have hundreds of nozzles for printing at 300 dots per inch or more. As energization signals are provided to the ink ejection elements as the printheads are scanned across the medium, ink droplets are ejected from the nozzles to create a pattern of ink dots to print text or an image.
Inkjet printing devices may include several print modes. In a unidirectional print mode, a printhead of the printing device prints just when it is moving in a given direction past the media in question, such as a sheet of paper. For example, the printhead may print just when it moves from left to right, and not when it moves from right to left. By comparison, in a bidirectional print mode, a printhead of the printing device prints when it is moving in either direction past the media. For example, the printhead may print both when it moves from left to right, as well as when it moves from right to left.
Bidirectional print modes include a one-pass bidirectional print mode and a multi-pass bidirectional print mode. A one-pass mode is one in which a pen or printhead passes a single time over a horizontal band of the print medium, and then the print medium is typically advanced by a distance that corresponds to the printhead height. A multi-pass mode is one in which a pen or printhead passes more than once over a horizontal band of the print medium. In each pass, the printhead deposits a swath having only a fraction of the total ink used in each section of the image, with areas being left unprinted to be filled in during one or more other passes. For each pass in multi-pass printing, the print medium is typically advanced by a distance that corresponds to the printhead height divided by the number of passes to be undertaken.
One-pass bidirectional print modes typically provide the best printing speed. These print modes, however, can produce image quality defects when printing area fills. One image quality defect in one-pass bidirectional print modes when printing medium or dark gray area fills is called gray scale shift banding or darkness shift banding, which shows up as a change in gray scale level in consecutive passes. Gray scale shift banding is typically very visible in dark gray area fills when printing in one-pass bidirectional print modes.
In one-pass bidirectional print modes, since the arrangement of the printheads in the scanning carriage is fixed, ink drops typically fall in a different sequence when the carriage is moving from left-to-right than when moving from right-to-left. For example, to create a dark gray color, some printers will use a combination of photo black ink drops and matte black ink drops. When the carriage moves from left-to-right, the matte black dots fall on top of the photo back dots. When the carriage moves from right-to-left, ink drops fall in the inverse sequence. Water in ink drops is immediately absorbed by the media and, therefore, the colorant is laid on the surface instantaneously. When matte black drops fall on top of the photo black drops, there is no time for the colorants to mix properly. In this way, matte black colorant will lay on top of the photo black when printing from left to right and vice versa when printing in the opposite direction. This results in gray scale shift in consecutive passes when printing some gray area fills.